Cloth Simulation - Sleeves keep sliding off arms
It's difficult to tell without seeing it animated or seeing more settings... however, this is likely an issue with the Collision settings on the model mesh.
Firstly, ensure you have 'Applied Scale' on all of your meshes - this is good practice anyway but if you haven't done this it can cause strange effects with your simulation.
Next check the Collision 'Cloth and Soft Body' settings. Reduce the 'Inner' and try the simulation again (free and re-bake) to see if that helps.
The 'Inner' boundary needs to be less than half the thinnest part of your mesh that's involved in the collision - otherwise the collision bound can &quot;leak&quot; out of the back face of your mesh and upset the collision.

After reviewing the uploaded Blend file, the issue was actually just that the Modifier stack was in the incorrect order :

In the original file the Collision modifier was above the Armature modifier. This meant that the collision was working correctly but was on the unposed model - which was obviously in a different position to the 'base' model.
Simply swapping the Collision to be after the Armature modifier resolves the problem and the collision is on the posed mesh.